Particulate silica produced by the flame hydrolysis process is characterized by fineness, high purity and high dispersibility and finds typical use as (1) a thickening, anti-settling or dispersing agent for liquids, (2) a reinforcing filler for elastomers and polymers, and (3) an anti-caking or flow-improving agent for powder.
In particular, particulate silica is indispensable to silicone resins because it exerts the above-mentioned characteristics to the full extent when combined with silicone resins.
Particulate silica by the flame hydrolysis process is generally produced by mixing tetrachlorosilane with hydrogen and air and burning the mixture as disclosed in JP-B 47-46274. It is convenient for the silicone manufacturer if the process starts with, instead of tetrachlorosilane, organohalosilanes such as methyltrichlorosilane which is a by-product in the synthesis of dimethyldichlorosilane.
The synthesis of particulate silica from organohalosilanes is known from JP-B 56-38526 corresponding to U.S. Pat. No. 4,108,964, JP-B 58-54085 corresponding to GB 2,049,641 and Japanese Patent No. 2889202 corresponding to EP 790213A. As noted in these patents, the process starting with organohalosilanes has the problem that silica is blackened due to residual carbon unless conditions are strictly controlled.
Yet, the organohalosilane itself is flammable and has a propensity that as the flame is lengthened, the flame temperature distribution locally varies, inviting a substantial size variation of silica particles and even formation of giant particles. When such particulate silica is used as a filler in silicone rubber, unexpectedly, the physical properties such as strength of rubber moldings are not adversely affected. However, the transparency of rubber moldings is degraded. For silicone rubber, the outer appearance including transparency is one of important factors for its commodity value.
The above-listed patents describe that particulate silica free of blackening due to residual carbon and having a specific surface area of 100 to 400 m2/g can be synthesized from organohalosilanes, but do not refer to the synthesis of silica having a specific surface area of 100 to 400 m2/g and a narrow particle size distribution of primary particles and ensuring the transparency of silicone moldings filled therewith.